Studies have continued on the unique phosphoprotein, pp17, which undergoes rapid phosphorylation in HL60 promyelocytic leukemia cells in response to treatment with phorbol ester (TPA). We have identified the non-phosphorylated form of this protein as a major cytosolic protein of Mr 18.4K, pI 5.9, and have named it 'prosolin'. Within 15 min of treatment of HL60 cells with TPA nearly 50% of pre-existing prosolin is phosphorylated, making this event one of the earliest and most significant biochemical changes resulting from TPA treatment in HL-60 cells. Studies with peripheral blood lymphocytes and malignant lymphoid cell lines suggest that synthesis of prosolin is associated with rapid growth of hematopoetic cells, while phosphorylation of prosolin may be associated with rapid cell responses associated with reduction of DNA synthesis and expression of genes involved in differentiated cell function. Amino acid sequencing and cDNA cloning of prosolin are in progress.